An LED is typically driven by a driver circuit comprising an AC-DC or a DC-DC converter delivering at least one regulated output, allowing generating and supplying relatively constant currents—in response to varying input voltages or to any other disturbances, such as variations of the load impedance that may result from variations of temperature or ageing of the load.
The Solid State Lighting (SSL) Industry's demand for small and compact power management units for LEDs is increasing, which statement is even stronger in the field of Power LEDs, for which energy from the power supply has to be delivered in the form of a constant current as efficiently as possible. Ideally, LED drivers comparable in size to the LEDs themselves would represent a significant breakthrough enabling new lighting concepts. Such a solution will require a system with a high level of reliability and efficiency, in order to fit the requirements of life-time, size and heat dissipation.
Known LED drivers are generally currently based on two main technologies: Linear Drivers and Switched Mode Power Supplies (SMPS).
Linear Drivers for DC-DC conversion have the advantage of offering high integration capabilities at low cost, owned to the fact that such drivers may only consist of long latency semiconductor switches and resistors which can be monolithically integrated with mature technologies; on the other hand, solutions based on linear drivers can have a poor efficiency whenever the voltage difference between input and output becomes high, either due to design tolerances, or by disturbances in the system such as variations of the load impedance that may result from variations of temperature or ageing of the load, or both.
The Current-Voltage characteristics of LEDs, as illustrated in FIG. 1 described in detail hereinafter, behaves abruptly when the voltage reaches a certain forward voltage threshold, usually designated by VF. Variations on the forward voltage threshold, which can be designated by ΔVF, for a given LED, are typically in the order of ±10% and therefore lead to significant power losses, which represent a challenge for the power management unit of an LED driver. In practice, linear drivers can only be used for sub-Watt applications.
SMPS drivers using inductive storage allow achieving better efficiencies; however the monolithic integration of all SMPS elements (in particular the reactive components, and more particularly inductors) is not yet mature enough to meet the requirements of the considered application.
SMPS drivers using capacitive energy storage—SMPS notably including Switched Capacitor Converters (SCC) or Charge Pumps (CP)—can provide highly efficient DC-to-DC voltage conversion with only the use of capacitors and semiconductor switches. Therefore capacitive-type SMPS can be more easily integrated compared to inductive-type SMPS. Using CPs in drivers for LEDs is known, and some available off-the-shelf integrated circuits include charge pumps. However all these products basically consist of a monolithic chip that integrates a CP powertrain and a linear regulator with the associated control, these products being broadly used in mobile phones backlighting, typically for powers up to one watt. In these solutions the input voltage, typically supplied by a Lithium-Ion battery, is first stepped-up or stepped-down to the closest voltage above the forward voltage of the LED. These chips provide current regulation by means of a linear driver, which in most cases translates to poor efficiencies, typically below 80%.
There is no known solution for mid to high power LED drivers that provides a compromise between a small volume, typically equivalent to the volume of the LED itself, and minimized losses due to variations of the forward voltage of the LED.